In digital circuits, clock signals are required to provide synchronization between various operations. Crystal oscillators are often employed as highly stable reference frequency sources to generate digital clock signals.
For many applications, it is necessary to operate a digital circuit at more than one frequency. For example, during a period of time in which digital operations are not needed, the frequency of the digital clock may be slowed down in order to save power.
Multiple crystal oscillators may be used to generate clock signals at a variable frequency. However, as crystal oscillators are expensive devices, it would be desirable to produce independent digital clock signals of various frequencies using a single crystal oscillator.
Also, it would be desirable to provide a digitally programmable circuit for generating digital clock signals of various programmable frequencies.